This invention relates to highly oriented oxide superconducting films. The invention further relates to processing of metal oxide- and metal fluoride-containing films into oxide superconductor films.
The discovery of superconducting ceramic oxides has fueled a tremendous effort to fabricate these oxides into high performance films and coatings. High temperature superconducting (HTSC) film fabrication methods can be largely divided into two areas: physical and chemical methods.
Physical methods include reactive evaporation, magnetron sputtering, e-beam deposition and laser ablation. While physical deposition methods form high quality films, these deposition techniques typically have very slow formation rates, and require high vacuum environments so that they require expensive equipment. In addition, the techniques are best suited for thin-film fabrication. For these reasons, physical deposition methods are extremely difficult to scale up to multi-meter lengths required for electrical or magnetic applications.
Chemical methods are largely based upon thermally activated chemical reactions of precursor compounds during film formation. Chemical film fabrication methods involve a precursor which is deposited onto a substrate and later transformed through thermal and chemical means to a film having the desired composition and phase.
Films may be prepared using metalorganic chemical vapor deposition (MOCVD), in which precursor films are deposited from metalorganic precursors having a high vapor pressure. Metal-organic solution deposition (MOD) processes involve the deposition of a precursor film from a condensed phase precursor. The precursor film is then heated and converted into the final ceramic in a separate heat treatment.
MOD processes are widely used in industry for the deposition of ceramic films. The process is ideally suited for the rapid, inexpensive deposition of films on large or continuous substrates. Other advantages of the MOD process include easy control of metal composition and homogeneity, short processing time, low capital equipment cost and low precursor cost.
Typically in MOD processes, metal carboxylates of carboxylic acids, alkoxides, or partially hydrolyzed alkoxides are dissolved in organic solvents and the resultant solution is deposited onto a substrate by dipping or spin coating. The precursor films produced by these coating processes are transformed into metal compound-containing coatings by heat treatment, which most commonly includes a series of distinct heating steps. While chemical methods represent versatile and inexpensive methods of film fabrication with potential for high speed production, they are very sensitive to secondary reactions which may be deleterious to final superconducting properties. For example, in the deposition of materials such as YBa2Cu3Oy, such processes are highly susceptible to the intermediate formation of barium carbonate (BaCO3). The stability of BaCO3 requires high processing temperatures ( greater than 900xc2x0 C.) and extended processing times in order to decompose the barium carbonate and obtain the oxide superconductor. The extreme reaction conditions result in film reaction with the substrate, poor texture of the oxide superconductor and incomplete formation of the oxide superconductor phase.
Chan et al. in Appl. Phys. Lett. 53(15):1443 (October 1988) discloses a hybrid process, known as an ex situ process, which includes the physical deposition of a precursor film which is then processed outside of the physical deposition chamber by conventional chemicothermal processes. This PVD process (BaF2 ex situ process) separates the deposition and conversion steps. This process involves codeposition of CuO, Y2O3, and BaF2 in the correct stoichiometric uniformly on the substrate. The film is then converted under conventional heating conditions into the oxide superconductor by annealing in the presence of water vapor. The limitations of physical deposition methods described above remain, however. Chan et al observed that improved electrical performance was obtained by increasing the PO2 and decreasing the PHF during the anneal step.
Cima et al. in U.S. Pat. No. 5,231,074, report the MOD preparation of Ba2YCu3O7-x (YBCO) oxide superconductor films having improved electrical transport properties by MOD using metal trifluoroacetates on single crystal SrTiO3 and LaAlO3. The films of a thickness of about 0.1 xcexcm possessed critical transition temperatures of about 90K and zero field critical current densities of greater than 106 A/cm2 at 77K.
In addition, the superconducting performance of epitaxial Ba2YCu3O7-x films prepared using the process described in U.S. Pat. No. 5,231,074 has been found to depend on film thickness. Electrical performance drops off dramatically as film thickness increases from 0.1 xcexcm to 1.0 xcexcm. Although thinner films have routinely been prepared with critical current densities greater than 106 A/cm2, application of conventional chemical processing techniques in the preparation of films with a thickness near 1.0 xcexcm never yielded results close to this level of performance. For example, a MOD process using metal trifluoroacetates has been used to prepare thin (70-80 nm) YBa2Cu3Oy (YBCO) films (where y is a value sufficient to impart superconductivity at temperatures of at least 77K) with Tc greater than 92K and Jc greater than 5xc3x97106 A/cm2 (77K, self field); however, it has not been possible to prepare much thicker films possessing similar properties. Indeed, prior to the development of the processing techniques described in this patent application no solution-based deposition process had been demonstrated that produced high Jc films with thicknesses of over 0.5 xcexcm.
Thicker oxide superconductor coatings are needed in any application requiring high current carrying capability such as power transmission and distribution lines, transformers, fault current limiters, magnets, motors and generators. Thicker oxide superconducting films are desired to achieve a high engineering (or effective) critical current (Jc), that is, the total current carrying capability divided by the total cross sectional area of the conductor including the substrate.
It is desirable that oxide superconducting coatings greater than 0.5 xcexcm in thickness have high critical current densities. There is a need for fabrication techniques which may be used to prepared these thick oxide superconductor films and coatings with superior electrical performances.
It is an object of the invention to provide oxide superconductor films having superior electrical properties.
It is a further object of the invention to provide oxide superconductor thick films possessing high epitaxial alignment, and preferably c-axis epitaxial alignment.
It is another object of the present invention to provide a method of processing metal oxyfluoride precursor films into high quality oxide superconductor films.
It is a further object of the invention to provide a method of fabrication for high quality relatively thick film oxide superconductors.
These and other objects of the invention are accomplished by controlling the reaction kinetics for the conversion of the metal oxyfluoride into an oxide superconductor, so that the rate of conversion takes place at a desired controlled rate. In particular, reaction conditions are selected which control the rate of consumption of BaF2 and/or other metal fluorides and thus the HF evolution rate which among other effects permits sufficient time for the transport of HF from the film and which also reduces the HF concentration during the nucleation of the oxide superconductor layer at the substrate/film interface. In particular, the reaction temperature and the moisture content of the processing gas used in the reaction are controlled so as to adjust the conversion rate of the metal oxyfluoride into the oxide superconductor.
The present invention is applicable to any chemical processing system which generates hydrogen fluoride upon hydrolysis in the preparation of a metal oxide. The presence of fluoride in the precursor film may have the additional advantageous effect of doping the product oxide superconductor with fluorine which had been demonstrated to increase its critical transition temperature and, hence, possibly its critical current density. The present invention may be applied to any film fabrication method which consumes barium fluorides or other metal fluorides during deposition and processing.
By xe2x80x9cmetal oxyfluoridexe2x80x9d as that term is used herein it is meant a composition which contains metals, oxides and fluorides. The composition may include cationic metallic species bound to both oxygen and fluoride, e.g., MOxFy, where x and y are selected to satisfy metal valency, or it may include a mixture of metal oxides and metal fluorides, e.g., MOx and MFy.
xe2x80x9cMoisture contentxe2x80x9d as that term is used herein, refers to the vol % water vapor contained in the processing gas used in the heat treatment of the invention at the point of its introduction into the furnace and may alternatively be referred to as PH2O or relative humidity (RH). Relative humidity may be referred to relative to a particular temperature since the capacity of the processing gas to contain water vapor is temperature-dependent. Moisture content is defined herein in terms of relative humidity (RH), which represents the amount of water (%) in the processing gas relative to the amount of water in the processing gas at maximum capacity (saturation) at the point of its introduction into the furnace at room temperature (RT).
By xe2x80x9ccoated conductorxe2x80x9d as that term is used herein, it is meant, a superconducting wire or tape in which the superconducting material is coated on the exterior of a substrate that forms the bulk of the wire or tape, or other article.
In one aspect of the invention, a method for preparing an oxide superconductor film includes providing a metal oxyfluoride film on a substrate, said metal oxyfluoride film having a thickness greater than or equal to about 0.5 xcexcm and comprising the constituent metallic elements of an oxide superconductor in substantially stoichiometric proportions; and converting the metal oxyfluoride into the oxide superconductor at a rate of conversion selected by adjusting a reaction parameter selected from the group consisting of temperature, PH2O and combinations thereof, such that an oxide superconductor film having a transport critical current density of greater than or equal to about 105 A/cm2 at 77K, zero field is obtained.
In another aspect of the invention, an oxide superconductor film is prepared by providing a metal oxyfluoride film on a substrate, said metal oxyfluoride film comprising the constituent metallic elements of an oxide superconductor in substantially stoichiometric proportions; and converting the metal oxyfluoride into the oxide superconductor in a processing gas having a moisture content of less than about 100% RH as determined at 25xc2x0 C.
In yet another aspect of the invention, an oxide superconductor film is prepared by providing a metal oxyfluoride film, said metal oxyfluoride film comprising the constituent metallic elements of an oxide superconductor in substantially stoichiometric proportions; and converting the metal oxyfluoride into the oxide superconductor under reaction conditions selected to provide an atmosphere above the substrate comprising an HF concentration at a level to provide an oxide superconductor film having a transport critical current density of greater than or equal to about 105 A/cm2 at 77K, zero field.
In yet another aspect of the invention, an oxide superconductor film is prepared by (a) providing a metal oxyfluoride film on a substrate, said metal oxyfluoride film comprising the constituent metallic elements of an oxide superconductor in substantially stoichiometric proportions; (b) converting the metal oxyfluoride into the oxide superconductor in a processing gas having a moisture content of less than 100% RH as determined at 25xc2x0 C. for a time sufficient to form a layer of the oxide superconductor at the substrate/film interface; and (c) completing conversion of the metal oxyfluoride into the oxide superconductor in a processing gas having a moisture content greater than that in step (b). In preferred embodiments, a time sufficient to form a layer of the oxide superconductor at the substrate/film interface is in the range of about 15 minutes to about 2 hour.
In preferred embodiments, the moisture content comprises a relative humidity less than about 95%, and preferably less than about 50%, and more preferably less than about 1-3% as determined at 25xc2x0 C. The substrate may comprise a metal or a ceramic, wherein the ceramic is selected from the group consisting of SrTiO3, LaAlO3, zirconia, preferably stabilized zirconia, MgO and CeO2. The substrate may be substantially lattice-matched with the oxide superconductor. In other preferred embodiments, the methods above further comprise annealing the oxide superconductor film so as to oxygenate the oxide superconductor.
In other preferred embodiments, conditions for converting the metal oxyfluoride comprise heating the metal oxyfluoride film in a processing gas having a moisture content of less than about 95-100% RH as determined at 25xc2x0 C. and at a temperature in the range of 700-900xc2x0 C., or heating in an environment where oxygen content is selected to be as low as possible at a given temperature while still maintaining stability of the oxide superconductor phase.
In preferred embodiments, the metal oxyfluoride film is deposited using a technique selected from the group consisting of MOD, MOCVD, reactive evaporation, plasma spray, molecular beam epitaxy, laser ablation, ion-beam sputtering and e-beam evaporation, or by depositing a metal trifluoroacetate coating onto the substrate and decomposing the metal trifluoroacetate coating to form the metal oxyfluoride film. Multiple layers may be applied. In preferred embodiments, the oxide superconductor film preferably has a thickness of greater than or equal to 0.8 microns (xcexcm), and more preferably has a thickness of greater than or equal to 1.0 micron (xcexcm).
In another aspect of the invention, an oxide superconductor article is provided in which an oxide superconductor film has a thickness of greater than 0.5 microns (xcexcm) disposed on a substrate and the article has a transport critical current density (Jc) of greater than or equal to about 105 A/cm2 at 77K, in zero applied magnetic field.
In yet another aspect of the invention, a coated conductor article, is provided including a metallic core; a buffer layer disposed on the core; and an oxide superconductor coating having a thickness greater than or equal to about 0.5 xcexcm, said crystalline buffer layer substantially lattice-matched with the oxide superconductor, said coated conductor exhibiting a critical current density of greater than or equal to about 105 A/cm2 at 77K, self field.
The article may be further characterized in that the article possesses a critical transition temperature (Tc) of greater than 92K. The article may be further characterized in that the oxide superconductor comprises a sufficiently high volume percent of c-axis epitaxy so as to provide Jc values of equal to or greater than 105 A/cm2 at 77K, in zero applied magnetic field. The article may be further characterized in that the oxide superconductor comprises residual fluoride so as to provide Tc values greater than 92K.
In preferred embodiments, the oxide superconductor coating has a thickness greater than or equal to about 0.8 xcexcm and more preferably greater than or equal to about 1.0 xcexcm. In other preferred embodiments, the conductor has a critical current density of greater than or equal to about 106 A/cm2 at 77K, self field. In other preferred embodiments, the oxide superconductor is characterized by a high degree of c-axis epitaxy.
In yet another aspect of the invention, an oxide superconductor article is provided including an oxide superconductor film having a thickness of greater than 0.5 microns (xcexcm) disposed on a substrate, said oxide superconductor being substantially c-axis epitaxially aligned.